Heretofore, as a method for widening a strand, there are known a method in which a water current or high-pressure air flow hits a reinforcing fiber strand to spread the constituting fibers in the width direction, a method in which a strand is vibrated by ultrasonic waves or the like to be widened in the air or liquid, and a method in which a strand makes contact with a widening jig to be extended and widened.
For example, Patent Document 1 or Patent Document 2 is known as the method of using the water current or high-pressure air flow. However, there is a problem that if water is used for fluid, large energy is required for a drying process after the strand is widened. When using absorption high-pressure air flow, a large number of supplementary facilities are undesirably needed due to scale-up resulting from the use of multiple spindles or speed up. In contrast, as in Patent Documents 3 to 5, the method of vibrating and widening the strand may be implemented by a relatively small device. However, the method of using such a vibrating jig is problematic in that a frequency is insufficient if a specific line speed is exceeded, so that it is impossible to obtain a sufficient width of strand.
Therefore, industrially, the method of making the widening jig contact with the strand is advantageous in that facility investment is relatively small and efficient production is possible. For example, Patent Document 6 discloses a method of uniformly and sufficiently widened reinforcing fiber strand by using a curved bar with a convex curved surface and a curved bar with a concave curved surface. However, this is problematic in that the tension of the strand is changed in the process, so that a center of the strand deviates from a center of the convex curved surface, and thereby the strand is non-uniformly widened.